October 19, 2009 at 6:02 am (dragonflies and damselflies, geology)
Tags: bedrock, Bruce Peninsula, dolomite, dolomite prairie, Hines emerald, Niagara formation, Somatochlora hineana
by Carl Strang
Bedrock is the kind of stone found closest to the surface at a particular point on the Earth. In DuPage County our bedrock is a sedimentary rock of Silurian age called dolomite, and it belongs to the Niagara formation. A bit over 400 million years ago this part of North America was a reef-dotted shallow part of the world ocean. Over a period of millions of years, precipitates and microscopic shells, along with some larger life forms, settled to the bottom of the sea and built up a layer of sediment that later solidified into limestone, or calcium carbonate. Later, some of the calcium became replaced with magnesium atoms, changing the rock enough chemically that it was less soluble, and worthy of a new name, dolomite.
DuPage County is part of a ring of Niagara formation bedrock that extends up the west coast of Lake Michigan, forms Wisconsin’s Door Peninsula and the southern boundary of the U.P., wraps around the Canadian side of Lake Huron to divide the main lake from Georgian Bay, continues south to form the Bruce Peninsula jutting into southern Lake Huron, and eventually wraps around northern Ohio and Indiana back into northeast Illinois. Niagara Falls pours over an erosion-resistant edge of this formation, which also has outliers in Missouri and Iowa.
A few years ago I drove around Lake Huron. One of my stops was the tip of the Bruce Peninsula. There, the bedrock is at the surface. It’s possible to get a sense of what our landscape might look like in northeast Illinois if it were not covered by glacial deposits. Instead of being surfaced with crushed dolomite, our trails might run over the raw rock.
A trip to the beach would look quite different.
We might find cliffs.
Though dolomite is not as subject to solution and cave formation as limestone, sea caves might occur where waves pound the shore.
We might even find structures like the “flowerpots” that stand on Flowerpot Island off the Bruce Peninsula tip.
But as it is, there are few places in northeast Illinois where the bedrock reaches the surface.
For example, the dolomite prairies near the Des Plaines River offer rare habitat for the federally endangered Hines emerald, a dragonfly that can live only where Niagara formation dolomite provides the right water chemistry for its crayfish-tunnel-dwelling larvae.
September 23, 2009 at 6:26 am (geology)
Tags: Agawa, bedrock, Canadian shield, continental glacier, glacier, granite, Green Bay lobe, greenstone, hematite, Illinois Beach State Park, Lake Maxinkuckee, Lake Michigan lobe, Muskallonge Lake State Park, Upper Peninsula, Whitefish Point
by Carl Strang
A second goal of my trip to Michigan’s Upper Peninsula last week was to investigate further the stones left by the most recent continental glacier. As I outlined in an earlier series of posts, my vacation trip last year was a pilgrimage into Canada to trace the route of the Lake Michigan lobe of that glacier, which is responsible for the deposits which cover the land in the northeast corner of Illinois. The turquoise line in the map below follows the route I think that lobe followed.
I studied the various categories of bedrock northeast of Lake Superior, chunks of which were picked up by the glacier and now reside where that powerful river of ice left them when it melted away. I found that there appeared to be commonalities in the stones left as drift along the Lake Michigan lobe’s route in Canada, on the Upper Peninsula of Michigan, and in northeast Illinois. Stones northwest of that route in Canada seemed different, and I was curious to see if those differences might hold farther south along the route of the Green Bay lobe, which is the one immediately west of the Lake Michigan lobe. I chose to visit Muskallonge Lake State Park, on the U.P.’s north shore, approximately in the center of the Green Bay lobe’s route, so that I could compare the beach stones there to those at Whitefish Point, at the U.P.’s tip, which was on the route of the Lake Michigan lobe.
It was a foggy day, but a few people were there. Some were gathering stones, a practice which might bias the results.
For instance, it seemed to me that beach stones at Canada’s Agawa Bay, along the Green Bay route, included an unusual number of red granites and greenstones. If these are selectively removed by visitors, the remaining stones might not represent what had been there originally. I certainly found greenstones, and in the following photo two appear.
However, there were very few compared to Agawa Bay. Here is a typical aggregation of Muskallonge stones, representing the Green Bay lobe.
Here is a corresponding photo for Whitefish Point, along the Lake Michigan lobe’s route.
While to my eye there did seem to be more reds and a few more greens at Muskallonge, and a few more grays and browns at Whitefish point, I don’t think the differences would hold up in a proper sampling procedure and statistical analysis. Furthermore, when I bring in a photo from Illinois Beach State Park (below), I am hard pressed to say that it is closer to one U.P. site or the other.
Nevertheless, the two years’ travel and study were enjoyable, and I learned a lot especially from studying the Canadian bedrock. The glacial drift may not provide additional support for the route map shown above, but the scratches on bedrock indicated by the little arrows in the geologists’ original map certainly are consistent with the turquoise line I added after last year’s trip.
Incidentally, there were places at Muskallonge Lake where there were deposits of black sands, I suspect composed of hematite like I found at Lake Maxinkuckee last winter.
The next installation from this trip will be more biological.
November 15, 2008 at 12:51 pm (geology)
Tags: bedrock, continental glacier, dolomite, geology, glacier, Niagara dolomite
by Carl Strang
This year on my vacation I went in search of the route followed by our lobe of the glacier. Our land surface was shaped by the most recent continental glacier, which melted away 18,000 years ago or so. Continental glaciers often are depicted as enormous masses of ice, but in fact they are a radiating group of ice rivers, flowing shoulder to shoulder out from their source area. Maps of the glacier are readily available showing general directions of flow, but I wanted to see if I could get a more detailed picture especially of the northern part of the route followed by our glacial lobe for northeastern Illinois. The maps I found were ambiguous on that point.
The last part of that glacier lobe’s route is clear enough: it followed the length of Lake Michigan. It was bounded to the west by the Green Bay lobe, the two divided by a ridge of bedrock, the Niagara dolomite of Silurian age, which also is our bedrock in DuPage County. Lake Michigan once was a river, but it was on relatively soft shale of Devonian age, which earlier glaciers progressively had gouged out on their journeys south. Now it was a convenient channel for the Lake Michigan lobe of the most recent (“Wisconsin”) glacier. When the lobe reached the south end of the lake basin it was confronted by Niagara dolomite, which wraps around the south end of the lake. The push and give of lobes on either side, along with the continued forced advance from the North, resulted in our glacial lobe cresting the dolomite and spreading out in a southwesterly direction from the lake basin, passing over Cook and Lake Counties on its way into DuPage.
When you look at chunks of rock left behind when the glacier eventually melted, you find much more than Devonian shale (nearly all of which was ground up to give us our wonderful clay soil) and Silurian dolomite (shown here outcropping near the tip of the Bruce Peninsula of Lake Huron in Ontario).
There are granites of assorted colors,
Granites found in DuPage County
gneisses in varied patterns,
Gneisses found in DuPage County
Schists found in DuPage (say that one carefully!)
diabase, basalt, quartzite, greenstone and iron ore pieces.
Iron Ore found in DuPage County
I wanted to find the source of these rocks. I headed to Canada. None of these odd igneous and metamorphic rocks originated in the U.S. The bedrock of the Upper and Lower Peninsulas of Michigan, which flank the north end of Lake Michigan, is Paleozoic in age and all sedimentary, including the segments of the Niagara dolomite ring which form the Door Peninsula of Wisconsin and part of the U.P. The igneous and metamorphic foreigners are literally so, having been carried by the glacier down from the land of hawckey, loonies and twonies (this is not intended to be mocking; we should have copied Canada’s use of coins for $1 and $2 denominations long ago).
So, the goal of this trip was a very generalized inquiry, seeking out the sources of those stones, trying to match them to their bedrock sources and in the process tracing the Lake Michigan glacial lobe’s route of flow.
To be continued…